1. Field of the Invention
This invention relates in general to apparatus for reproducing a digital signal in which a clock signal is generated from the reproduced signal.
2. Description of the Prior Art
Transmission systems for transmitting information having burst-shape, such as recording and reproducing apparatus are known in which a plurality of rotary heads are mounted on the periphery of a tape guide drum and have, for example, equal angular spacing as, for example, an angular spacing of 180.degree. between the heads. A magnetic tape is wrapped around the periphery of the tape guide drum at an angular spacing which is smaller than the angular range of 180.degree. and might, for example, be in an angular range of 90.degree.. An example of such recording and reproducing apparatus is disclosed in European patent application No. 0092403 which was filed by the assignee of the present application and published on Oct. 26, 1983.
In this prior art recording and reproducing apparatus, a radio frequency signal is reproduced by the rotary head during the normal playback mode and has a signal waveform of a burst-shape which has a high signal level as illustrated in FIG. 1A. This is because signals recorded on the tape are reproduced only when the rotary head is substantially in contact with the tape. The RF signal reproduced by the rotary head is waveform-equalized by a waveform equalization circuit and is waveform-shaped by a waveform shaping circuit and then fed to a phase locked loop circuit. The phase locked loop circuit is provided so as to generate a clock signal from the reproduced digital signal.
When the level of the RF signal is sufficiently large the voltage controlled oscillator (VCO) of the PLL circuit is locked and placed in a stable state as illustrated in FIG. 1B. When the level of the RF signal is, on the other hand, very small which means that substantially no signal exists, the VCO of the PLL circuit is not in the locked condition and is in a free running state as illustrated in FIG. 1B so that the PLL circuit runs freely which means that the VCO runs freely. Thus, the oscillation frequency of the VCO will vary near the free running frequency and the unstable state will be maintained until a regular RF signal arrives again.
This is also true for high speed playback modes such as the fast forward (FF) search mode and the rewind (REW) search mode. Thus, upon the high speed playback mode the plurality of rotary heads scan a plurality of tracks during one scanning such that at that time each head generates an output when it scans the tracks whose azimuths are coincidence with the head and each head generates no output when it scans the tracks whose azimuths are not coincident with the head so that an RF signal which has a so-called bead-shape illustrated in FIG. 2A is obtained.
Thus, the PLL circuit to which such RF signal is supplied will be locked and placed in a stable state at a location on the tape where the level of the RF signal is sufficiently large, but it will not be locked as shown in FIG. 2B and will be in a free running state and become unstable at locations on the tape where no RF signal exists or at a trough portion of the RF waveform having bead-shape.
In prior art apparatus in which the level of the RF signal becomes small, the PLL circuit is released from the locked state and the PLL circuit is put into the free running state and under these conditions there are various defects of the apparatus.
First, the capture range (oscillation frequency range of the VCO in which the PLL can be locked to the input signal which is being varied from the initial state so that the PLL is not in a locked state) cannot be widened. Even if the capture range could be widened, the pull-in time of the PLL (time necessary for locking the PLL) becomes long and the locking range of the PLL (oscillation frequency range of the VCO in which the PLL can maintain the locked state to the input signal when the input signal is being varied from the initial state such that the PLL is in the locked state) is not substantially widened. At the present time, the lock range is about .+-.2 to 3% under best conditions. In addition, the adjustment of the free running frequency of the oscillator is required. Furthermore, even if the recovery time of the PLL (the time period from the time when the PLL is unlocked to the time when it is locked again) from a drop out of the RF is short then the PLL will be locked for a long period of time.
Also, in the prior art waveform equalization circuit, the waveform equalization characteristic for equalizing a waveform of a signal reproduced from the tape is fixed to that which occurs in the normal playback mode. As a result in the case of for example, a high speed playback mode or a variable speed playback mode such as when the relative speed between the tape and the head becomes faster or slower than the speed in the normal playback mode, the waveform equalization characteristic which is fixed to the normal playback mode cannot properly operate with the various playback modes because the frequency of the data is shifted upwardly or downwardly in response to the relative speed. This defect causes the data error rates to be increased.